Conventionally, for example, as disclosed in Japanese Laid-Open Patent Application 4-121260, a brake force control apparatus which generates, when an emergency braking is required, a brake force greater than that generated in a normal time is known. The above-mentioned conventional apparatus comprises a control circuit which generates a drive signal corresponding to an operational speed of a brake pedal and a fluid pressure generating mechanism which generates a brake fluid pressure corresponding to the drive signal generated by the control circuit.
The control circuit determines that, when an operational speed of a brake pedal is less than a predetermined value, the brake pedal is not normally operated. In this case, the fluid pressure generating mechanism is controlled so that a brake fluid pressure corresponding to a brake pressing force is generated. Hereinafter, this control is referred to as a normal control. Additionally, the control circuit determines that, when an operational force of the brake pedal exceeds a predetermined value, an emergency braking is required by the driver. In this case, the fluid pressure generating mechanism is controlled so that a brake fluid pressure is maximized. Hereinafter, this control is referred to as a brake assist control. Thus, according to the above-mentioned conventional apparatus, a brake force corresponding to a brake pressing force can be generated in a normal time, and a large brake force can be immediately generated in an emergency.
In the above-mentioned conventional apparatus, a normal braking operation and an operation requiring an emergency braking are discriminated in accordance with an operational speed of the brake pedal. Generally, the operational speed of the brake pedal when an emergency braking is required is higher than that of the normal braking operation. Thus, according to the above-mentioned discriminating method, the operation requiring an emergency braking and the operation requiring a normal brake can be discriminated with high accuracy.
However, for the purpose of obtaining a suitable deceleration, depending on travel circumstances, the brake pedal may be slightly pressed at a high speed without an intention to rapidly decelerate the vehicle. (Hereinafter, this operation is referred to as a small high-speed operation). In an apparatus in which the emergency braking and the normal brake are discriminated based on only an operational speed of the brake pedal such as in the above-mentioned apparatus, when the above-mentioned small high-speed operation is performed, it is possible that an erroneous determination is made that an emergency braking is required.
Additionally, in the above-mentioned apparatus, when the brake pedal is pressed at an operational speed exceeding a predetermined value, the fluid pressure generating mechanism is switched from a state for realizing the normal control to a state for realizing the brake assist control. Such a switching operation requires a certain time delay. Accordingly, when a brake fluid pressure at a high-pressure level can be obtained by continuing the normal control when a driver is highly skilled, it is preferred that the switching to the brake assist control not be performed.
However, in the above-mentioned conventional apparatus, when an operational speed of the brake pedal exceeds a predetermined speed, the switching to the brake assist control is always performed. In this regard, the above-mentioned conventional apparatus may give an unpleasant feel to the driver due to that control when the driver's skill level is high.
Additionally, depending on travel circumstances of the vehicle, there may be a case in which a braking operation is started gently and, thereafter, the brake pedal is pressed at a high speed, due to an emergency braking being required. (Hereinafter, such an operation is referred to as a spurt operation.) When the above-mentioned spurt operation is performed, a brake fluid pressure has already been increased to a certain level at a stage in which the brake pedal is pressed at a high speed. Accordingly, the operational speed of the brake pedal in the spurt operation is not as high as the operational speed of the brake pedal in an ordinary emergency braking.
However, in the above-mentioned conventional apparatus, it is always determined whether the braking operation by the driver is a normal braking operation or an operation requiring an emergency braking based on the determination as to whether or not the operational speed of the brake pedal exceeds the constant threshold value. Accordingly, the above-mentioned conventional apparatus has a characteristic in which the switching from the normal control to the brake assist control tends not to be performed when the brake pedal is subjected to the spurt operation.
As mentioned above, the above-mentioned conventional apparatus may generated a difference between a driver's intention and contents of the control to be performed, since the switching between the normal control and the brake assist control is performed based on the determination as to whether or not the operational speed of the brake pedal exceeds the constant threshold value.